1. Field of the Invention
Polyether polyurethane moldings are being increasingly used in the manufacture of automobiles and furniture and in home construction. Molded polyether polyurethane foams are especially important because they are lightweight and are resistant to moisture, weather, temperature extremes, and aging. As an illustration, molded, foamed polyether polyurethane elastomers have become of special interest in the manufacture of force-reducing impact media such as safety impact bumpers for automotive vehicles and impact resistant automotive fascia.
The high demand for molded polyether polyurethane articles requires that they be produced in the largest numbers in the shortest possible time. Polyurethane-forming mixtures are eminently suited for mass production because the reactants are liquid, that is, they are pumpable, and are quick-reacting. The problem has existed, however, in providing adequate mold release in the shortest possible time to take fullest advantage of the unique capabilities of the polyurethane systems.
As an illustration, it has been proposed that automotive bumpers and fascia be manufactured by pumping a polyether polyol and isocyanate prepolymer through a Krauss-Maffei mixing head where intimate mixing of the polyurethane components will occur in a matter of seconds. The mixed, uncured mixture will then proceed, under high pressure, to a mold cavity where the mixture will cure to a solid or micro-cellular elastomer. The cured polyurethane will be removed in short order to provide a clean mold capable of accepting additional mixture. Cycle times being discussed (from initial filling of the mold to ejection of the cured polyurethane) are in the order of 1 to 3 minutes. To achieve these production cycle times, the utilization of an internal release agent in the urethane ingredients is essential.
The field of this invention relates to the quick release of polyurethane compositions from molds in which they are formed and shaped.
2. Description of the Prior Art
Heretofore, release of molded articles from molds in which they have been formed has been achieved by coating the surface of the mold cavity with an agent which facilitates release of the molded article from the walls of the mold cavity. Procedures such as this are described in U.S. Pat. Nos. 3,694,530, 3,640,769 3,624,190, 3,607,397 and 3,413,390. This procedure has severe drawbacks. The agent, after molding, adheres either to the walls of the mold cavity or to the surface of the molded article or, in the usual case, to both. After multiple moldings and application of release agent, the agent tends to build up on the surface of the mold cavity walls and eventually covers and obscures any detail on the mold cavity surface desired to be imparted to the molded article. Also, the presence of release agent adhering to the surface of the molded article can impede subsequent operations on the article, such as painting or adhering operations. While it is possible to clean the surfaces of molded articles in preparation for painting or adhering operations, this adds to the time and expense of production. Additionally, the need to reapply the release agent after each molding or a limited number of moldings interrupts the molding operation and slows down output.
As an illustration, current practice in molding urethanes is to apply an externally applied substance to the mold cavity. The externally applied release agents are applied by spray or wipe-on techniques. Generally, the release agent adheres to the surface of the molded urethane, thus removing the deposit applied to the mold surface. This presents problems in that additional release agent has to be applied to the mold, and the surface of the urethane has to be degreased in order for paint to adhere to the urethane. Cycle times are long for such a process, which adds to the cost of manufacture of each urethane part.
U.S. Pat. No. 3,726,952 describes the use of additives for polyurethane foam forming mixtures for the purpose of imparting mold release properties. The additives described contain at least 25 carbon atoms and are salts of an aliphatic carboxylic acid and either a primary amine or an amine containing amide or ester groups. While these additives may provide the release property to some extent, they have not been found to provide effective release when used as the sole mold release additives. In addition, these additives contain fatty acid groups which cause cure interference to the urethane foam formulation.
U.S. Pat. No. 3,725,105 describes the addition of silicones (siloxanes) to epoxy coating compositions to impart a release surface to the coating after curing and thus prevent the adhesion of dirt and other foreign material thereto. However, the silicones of that patent include siloxanes containing a monovalent hydrocarbon radical or a halogenated monovalent hydrocarbon radical and do not encompass the carboxyalkyl siloxanes of the present invention.
While particularly unique organophosphorus internal mold release additives can be employed in polyurethanes as described in co-pending application Ser. No. 536,956 filed Dec. 23, 1974, now U.S. Pat. No. 4,024,088, there is room for still further improvement with respect to mold release additives.